1) Field of the Invention
The present invention relates to a production method of an expansion-molded article and a filling apparatus of foamed particles of a thermoplastic resin for use in such a method.
2) Description of the Related Art
Foams of thermoplastic resins have characteristics and properties such that they are light-weight and excellent in thermal insulating property and cushioning property. Therefore, the foams are widely used in various shapes or forms in thermal insulating materials, cushioning materials, packaging materials, etc. As a method of producing such foams, there has been widely known the foamed-in-place molding in which a thermoplastic resin is first of all impregnated with a blowing agent, the thermoplastic resin is expanded to produce foamed particles, and the foamed particles are then filled into a mold and heated with steam or the like, thereby expansion-molding them.
At the early time the foamed-in-place molding started to be used in industry, the foamed particles were fed under pressure by compressed air or the like to fill them in an amount as full as possible into the mold, thereby molding them. According to this method, the constant volume of molded articles can be obtained. However, such a method involved a drawback that the constant weight of molded articles can not be obtained because scattering of amounts of the foamed particles to be filled into the mold becomes extremely great due to scattering of pressures upon filling under pressure, densities and particle sizes of the foamed particles, etc. As described in Japanese Patent Application Laid-Open No. 27117/1987, it has accordingly been used to measure the volume of foamed particles by means of a metering device so as to always fill a fixed volume of the foamed particles into a mold.
According to the method wherein the volume of the foamed particles is measured to fill them, the constant volume of the foamed particles is always filled into the mold. It is therefore possible to provide the constant weight of molded articles so long as the foamed particles always have a fixed expansion ratio (density). Even if expansion of the thermoplastic resin particles are conducted under as much control as possible in the production process of the foamed particles, it is however difficult to entirely do away with the scattering of expansion ratios of the resultant foamed particles. In the method wherein the fixed volume of the foamed particles are always filled into the mold to mold them, the resultant expansion-molded articles therefore vary in density and weight due to scattering of the expansion ratios of the foamed particles. In conclusion, the method wherein the volume of the foamed particles to be filled is kept constant could not furnish a solution for the provision of always the constant weight of molded articles. With the manufacture of a variety of precise industrial products, there has been a demand for making the weight of every products uniform in recent years in particular (for example, core materials for automobile bumpers). The conventional method could not meet such a demand.